DC motors are used in a variety of applications. Several methods for varying the speed of the rotor are known, whether the motor has brushes or is a so-called brushless motor.
The speed of brushless motors may be varied by varying the supply voltage of the windings of the stator, for example, by reducing with a rheostat the voltage applied to the terminals of the motor. A motor may also be driven in a switching mode, that is, by coupling the motor terminals alternately to the supply rail and ground potential. This is normally done by using a half bridge stage formed by a pair of switches, commonly MOS transistors of opposite conductivity, driven in phase opposition. The effective voltage applied to the motor terminals is determined by the ratio between the duration of the phase in which the terminal is coupled to the supply voltage, and the duration of the phase in which it is grounded, according to a PWM mode.
A brushless motor has a permanent magnet rotor and a stator with a certain number of windings (most commonly three) that may be customarily connected in a star or a polygonal (triangle or delta) configuration. The motor is driven by coupling its windings to a supply node and to a ground potential according to a cycle (phase) excitation sequence.
In addition to the conventional star or polygonal configurations, the windings may alternatively be configured according to a so-called independent phases configuration, where both terminals of each phase winding are connectable to respective external driving circuits and driven independently from the other phase windings.
As it is well known, the revolution of the rotor induces a back electromotive force in the windings of electrical motors. Such a back electromotive force BEMF, under particular operating conditions, may cause voltage surges, i.e., relatively large overvoltages, on the supply rails. For example, such a condition is reached when a voltage smaller than the back electromotive force is applied to the spinning rotor for reducing its rotation speed.
A driving voltage that is obviously greater than the back electromotive force induced on the windings of the motor by the rotation of the rotor is normally applied to the windings of the motor for forcing a certain current through the phase windings of the motor, and therefore, produce a torque. When the voltage is lowered, the back electromotive force may become greater than the applied voltage and cause a voltage surge on the supply lines.
These voltage surges may disturb, sometimes in an unacceptable manner, the functioning of electronic circuits coupled to the same supply lines of the motor. According to well known techniques, a capacitor may be connected between the supply rail and the ground node for filtering the switching noise on the supply line, and if sufficiently large, also for limiting the amplitude of voltage surge peaks. Since these capacitors cannot be made as large as would be desirable because of cost and other factors, they are commonly provided with a Zener diode in parallel. When the voltage on the capacitor exceeds a certain value, the Zener diode turns on, thus discharging the capacitor.
This known approach does not satisfactorily eliminate cost and has other drawbacks. The Zener diode must necessarily be a power diode capable of absorbing current peaks of several amps for discharging the capacitor in relatively short times. Moreover, this known technique may only limit the maximum amplitude of voltage surges that remain observable.